The increasingly diverse demographic composition of the United States has significant implications for education, health care, the economy, politics, and other domains of social life. Our proposed research seeks to better understand how racial and ethnic diversity-the representation of multiple racial-ethnic groups in a population-manifests itself across geographies that range from states to places. Decennial census and American Community Survey data will be used to examine diversity magnitudes, structures, trends, and correlates throughout the national settlement system over three decades (1980-2010) for both general and detailed racial-ethnic categories. We study diversity from geocentric and group-centric orientations, with the spatial assimilation, ethnic stratification, and ecological perspectives providing a theoretical foundation. Our analysis has five specific aims. Aim 1: To describe diversity at the state and community scales, paying special attention to the geographic pervasiveness of the trend toward greater diversity. At the community level, we will employ the entropy index, a 'majority rule' compositional typology, transition matrices, cohort analysis, and growth mixture models to assess whether metropolitan, micropolitan, and rural areas and places are converging or diverging in diversity. Aim 2: To compare changes in the racial-ethnic diversity of states and communities to changes on other, non-racial dimensions of demographic diversity (e.g., age, socioeconomic status), asking whether a 'master trend' in societal differentiation is underway. Aim 3: To document the role of spatial processes in community racial and ethnic diversity. Moran's I, LISA, and transition matrices will help us determine how well a contagious diffusion model accounts for the spread of diversity across counties nationally and across places within selected metro areas. Aim 4: To identify the correlates of community diversity, both cross-sectionally and over time. Multivariate models for metro, micro, and rural areas and multilevel models for their constituent places will be estimated to document the associations between diversity and minority group standing, community ecological characteristics, and the racial mix of neighboring geographic units. Aim 5: To develop a group-centric approach to diversity by investigating the locational dispersion of particular racial- ethnic groups and their members' exposure to diversity. In each of the five aims, we emphasize those community units that approximate the governmental jurisdictions, service districts, and housing and labor markets where diversity-related issues often require policy attention.